High pressure reactors, such as tubular reactors and autoclaves, are used for the polymerization of ethylene at high pressure, for example, at pressures of over 1000 bar, and up to 3000 bar or even higher. In such processes, fresh ethylene from an ethylene supply together with recycled ethylene, optionally together with comonomer, is compressed in a series of compressors to the reactor pressure and is then introduced into the reactor where polymerization takes place, typically initiated by the compounds such as peroxides or oxygen. The compressed ethylene may be split into several streams, the first of which enters the reactor at one end (the front end) and the others (known as “sidestreams”) enter at points along the length of the reactor or, alternatively, all of the ethylene may enter the reactor at the front end. In order to start the polymerization reaction off, it is necessary to heat the first ethylene stream to a temperature at or above the “light off” temperature of the particular initiator or blend of initiators used. That heating is achieved by passing that stream of ethylene through a heater, known as a preheater.
In the reactor a portion of the ethylene is polymerized and a hot mixture comprising polymer and unreacted monomer leaves the reactor via an outlet and passes through a high pressure let down valve into a separation system in which the polymer is separated from unreacted ethylene gas. The unreacted ethylene gas is then treated in a recycle system to remove polyethylene waxes and to cool it before it is recompressed for return to the reactor. In many ethylene polymerization plants, the ethylene recycle system includes a boiler, known as a waste heat boiler, in which the heat from the recycled ethylene is used to boil water to produce steam, typically medium pressure steam, which is used to heat at least part of the preheater. In that way, some of the heat generated by the exothermic polymerization reaction can be used productively, thereby reducing the overall energy required to operate the plant with obvious economic and environmental benefits.
Typically, high pressure polymerization plants produce a range of grades of polymer varying in such characteristics as molecular weight, comonomer content, haze and density. One way of influencing the properties of the product polymer is by controlling the temperature profile in the reactor including the temperature at which the ethylene enters the reactor. Accordingly, the set temperature to which the ethylene is heated by the preheater will vary over a wide range, for example, from 130° C. to 220° C., depending on the particular grade to be produced at that time.
One known type of preheater comprises a first section of heating jacket which uses the medium pressure steam from the waste heat boiler and a second section which uses high pressure steam generated by a conventional boiler. Steam temperature is related to its pressure, and so the second section is at a higher temperature than the first section.
In order to produce certain grades of polymer requiring a very high reactor inlet temperature, the medium pressure steam in the first section of the preheater may be supplemented by the addition of high pressure steam. That addition succeeds in raising the pressure and temperature of the medium pressure steam and hence the heat input into that first section. However, the increase in pressure of the medium pressure steam also increases the boiling point of the water in the waste heat boiler and therefore reduces the temperature difference between that boiling water and the recycle ethylene, with a consequent drop in the efficiency of the waste heat boiler. That drop in efficiency has two undesirable consequences. Firstly, the amount of medium pressure steam generated is reduced and so more high pressure steam must be used in the preheater thereby increasing the fuel cost. Secondly, there is a decrease in the cooling of the recycle ethylene in the waste heat boiler and therefore there is an increased burden on the other heat exchangers in the recycle system, with a consequent increase in load on the plant cooling water.
For grades where low reactor inlet temperatures are required, no high pressure steam is used and parts of the first section of the preheater are blocked, again reducing the steam production of the waste heat boiler with the same two undesirable consequences mentioned above.
High pressure ethylene polymerization plants usually have a system for the generation and distribution of low pressure steam, typically referred to as a low pressure steam net. Low pressure steam has many uses, for example, heating wax knock-out pots in the ethylene recycle system to keep the contents molten. Depending on the location of the plant, in winter low pressure steam is often used to prevent water pipes from freezing, and therefore the demand for low pressure steam may rise in winter. Low pressure steam may be obtained by flashing off the condensate collected in the high pressure steam net. Although that method is economical, the generation of low pressure steam is thereby tied to the generation of high pressure steam and during times when the demand for low pressure steam outstrips the volume available from that route it is typically necessary to release some high pressure steam into the low pressure steam net, with a consequent increase in the cost of high pressure steam generation.